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Chapter 5: Other Relational Languages

Chapter 5: Other Relational Languages. Query-by-Example (QBE) Datalog. Query-by-Example (QBE). A graphical query language which is based (roughly) on the domain relational calculus Two dimensional syntax – system creates templates of relations that are requested by users

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Chapter 5: Other Relational Languages

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  1. Chapter 5: Other Relational Languages • Query-by-Example (QBE) • Datalog

  2. Query-by-Example (QBE) • A graphical query language which is based (roughly) on the domain relational calculus • Two dimensional syntax – system creates templates of relations that are requested by users • Queries are expressed “by example” • Microsoft Access supports a variant of QBE called Graphical Query By Example (GQBE)

  3. QBE Skeleton Tables for the Bank Example

  4. Queries on One Relation • Find all loan numbers at the Perryridge branch. • _x is a variable (optional; can be omitted in above query) • P. means print (display) • duplicates are removed by default • Find the names of all customers who have a loan from the Perryridge branch.

  5. The Condition Box • Allows the expression of constraints on domain variables that are either inconvenient or impossible to express within the skeleton tables. • Complex conditions can be used in condition boxes • E.g. Find the loan numbers of all loans made to Smith, to Jones, or to both jointly

  6. Datalog • Prolog-like logic-based language that allows recursive queries (based on first-order logic) • A Datalog program consists of a set of rules that define views. • Example: define a view relation v1 containing account numbers and balances for accounts at the Perryridge branch with a balance of over $700. v1(A, B) :– account(A, “Perryridge”, B), B > 700. • Retrieve the balance of account number “A-217” in the view relation v1. ? v1(“A-217”, B). • To find account number and balance of all accounts in v1 that have a balance greater than 800 ? v1(A,B), B > 800

  7. Syntax of Datalog Rules • A (positive) literal has the form p(t1, t2 ..., tn) • p is the name of a relation with n attributes • each tiis either a constant or variable • Comparison operations are treated as predicates • E.g. X > Y is treated as a predicate >(X,Y) • “>” is conceptually an (infinite) relation that contains all pairs of values such that the first value is greater than the second value • Arithmetic operations are also treated as predicates • E.g. A = B + C is treated as +(B, C, A), where the relation “+” contains all triples such that the third value is thesum of the first two

  8. Syntax of Datalog Rules (Cont.) • Rulesare built out of literals and have the form: p(t1, t2, ..., tn) :– L1, L2, ..., Lm. • each of the Li’s is a literal • head – the literal p(t1, t2, ..., tn) • body – the rest of the literals • A factis a rule with an empty body, written in the form: p(v1, v2, ..., vn). • indicates tuple (v1, v2, ..., vn) is in relation p

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